US Tax

1. If your husband becomes a US citizen, he is liable to US tax.

2. If he joins you after he has retired, he may still be subject to US tax depending on his income status.

3. If you have got a green card, he may be allowed to reside in US. However, please note the following:

Green card holders married to non-U.S. citizens are able to legally bring their spouses and minor children to join them in the USA. The foreign spouse of a green card holder must wait for approval of an 'immigrant visa' from the State Department before entering the United States. Due to numerical limitation on the number of these visas, the current wait time for approval is four to five years for all non-retrogressed countries (including Western Europe), and many more for the retrogressed countries. In the interim, the spouse cannot be legally present in the United States (let alone work), unless she secures a visa for herself using some other means. However, securing such a visa is usually difficult (and nearly-impossible at US embassies in some countries). This is because the spouse has to overcome presumed immigrant intent in order to qualify for a non-immigrant (or tourist) visa, a position at odds with her or his marriage to a US green card holder. Due to the long wait and the immigration intent issues, many green card holder opt to wait to become US citizens (usually 5 years), and only then sponsor their spouses and children (the process is much faster for US Citizens). This, however, imposes a family separation of several years, unique in the immigration laws of industrialized nations.
http://en.wikipedia.org/wiki/Permanent_residence_%28United_States%29

4. It is more convenient for your eldest son to study in US when he gets a green card.

5. Whether he is able to get a part-time job or not depends on his talents, qualifications, experiences, etc.

6. In view of the recent economic downturn in US and high unemployment rate there, you should take a serious consideration on this matter.